zebra: Allow runtime determination of v6 RR semantics

zebra: Allow runtime determination of v6 RR semantics

The linux kernel is getting the same Route Replace semantics
for v6 that v4 uses.  Allow the end-user to know if their
kernel has this ability and if so to specify it so zebra
can take advantage of this.

Why not do auto-detection?  Because you would have to write
code in zebra to add a route then add the same route again
with different nexthops to see if which semantics it is using.
It sure is easier to just add a cli that allows the user to
do it.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>